1. Field of the Invention
This invention relates to a signal comparing circuit and, more particularly, to a signal comparing circuit having a hysteresis circuit.
2. Description of the Prior Art
When a signal in the very high frequency band, such as an FM radio or a VHF television signal, is received by a receiver in a car, the signal's short wavelength and almost complete lack of a ground wave can cause a sudden decrease in signal strength if the vehicle is moved into a fringe area. The decrease in the strength of the signal received can be caused directly by a decrease in the strength of the available signal or by multipath interference. A decrease in signal strength causes deterioration in tone quality in radio signals (a problem particularly in stereo broadcasts) and deterioration in the picture quality in TV signals (causing snow, ghosts, synchronization disturbances, etc.).
A fringe area is typically one in which the signal strength is below 60 dB/m. When the receiver is moving, such as with a car, houses, metal poles, etc., along the road, multipath interference and fluctuations in signal strength occur through a range of .+-.20 dB/m. However, in those circumstances signal strength can also go as low as about 20 dB/m, which results in distortion in an FM stereo broadcast and disturbances in picture synchronization in a television broadcast.
To remove such defects, a so-called "diversity receiver" has been proposed in which two antennas are fixed to the vehicle. The antenna inputs are compared and the higher input signal level is detected and fed to the receiver. However, signals can be subject to momentary level fluctuations by, for example, residual AM-components or noise contained in the received signal. Because of such momentary fluctuations, the output of the detection circuit in the diversity receiver is frequently switched. Such frequent switchings can, in FM stereo broadcasts, cause audible switching noise and, in television broadcasts, that switching "noise" can also disturb the synchronization of the picture.
For this reason, signal comparing circuits such as that illustrated in the figure labeled "PRIOR ART" have been proposed. Such circuits use a differential amplifier for comparing signals received by two antennas and detecting the higher one and, to cope with spurious switching, are provided with a hysteresis circuit. The hysteresis circuit provides a "dead zone" in which a change in relative signal level will not change the detection output. Thus, in an FM receiver, the amount of audible switching noise is reduced and, in the case of a television receiver, noise and synchronization disturbances are reduced.
In the illustrated prior art signal comparing circuit 3, a differential amplifier 1 is supplied at its non-inverting input terminal with a first input circuit signal V.sub.1 applied to a first terminal t.sub.1 from a first antenna (not shown) through an input resistor R.sub.1 and at its inverting input terminal with a second circuit input signal V.sub.2 applied to a second terminal t.sub.2 from a second antenna (not shown). The amplifier output and the non-inverting input terminal are connected by a hysteresis circuit 2 having a feedback resistor R.sub.2. A power source 4 is also connected to the terminal t.sub.1 together with a signal source 5.
Now, assume that the amplifier input voltages V.sub.IN1 and V.sub.IN2 to the differential amplifier 1, derived from the circuit input signals V.sub.1 and V.sub.2, satisfy the condition V.sub.IN2 &lt;V.sub.IN1. Then, at the output of the differential amplifier 1 the higher level output voltage V.sub.H appears as the detection output V.sub.0. The non-inverting amplifier input is therefore applied with the bias ##EQU1## through the hysteresis circuit 2 and ##EQU2## is established. If the second input voltage V.sub.2 is then increased until V.sub.IN2 &gt;V.sub.IN1 is established, the detection output V.sub.0 appearing at the output of the differential amplifier 1 is changed to the lower level voltage V.sub.L. The non-inverting input terminal is then applied with the bias ##EQU3## through the hysteresis circuit 2 and ##EQU4## Now, if the circuit input voltage V.sub.2 is again lowered until V.sub.IN2 &lt;V.sub.IN1, V.sub.IN1 will return to its other level, as described above.
Accordingly, the level of the circuit input signal V.sub.2 will increase or decrease as compared with that of the circuit input signal V.sub.1, but the amplifier output changes only when V.sub.IN1 and V.sub.IN2 change relative to each other. Thus, a relative level range between the input signals V.sub.1 and V.sub.2 is provided within which the amplifier output will not change and that range, defined as the circuit "hysteresis width" V.sub.TH, is given as: ##EQU5## The equation (1) shows that, in the prior art signal comparing circuit 3, the hysteresis width V.sub.TH is constant regardless of the fluctuation of the circuit input signal levels (V.sub.1 and V.sub.2).
However, a constant hysteresis width V.sub.TH has a significant shortcoming. Relative signal strength can fluctuate enough to cause switching of the output level even though the same signal should be fed to the receiver. For example, an FM signal transmitted from a transmitter contains a residual AM component. And, in the receiver, the center frequency can deviate and, under certain circumstances, the AM component can be mixed into the FM signal. In addition, the amplitude of the undesired AM components is proportional to the signal level. Thus, if the hysteresis width or dead zone is constant regardless of circuit input signal level, spurious switching of the differential amplifier can occur.